(1) Siderophores are low-molecular weight organic compounds produced by microorganisms to facilitate iron-transport from the environment to the cell. A novel siderophore, PsX, produced by an unknown strain of Pseudomonas, was investigated by various spectroscopic methods. PsX was found to consist of four components having the same peptide backbone acylated with four different fatty acids. On the basis of the CID mass spectral analysis, a tentative sequence was assigned to the heptapeptide of PsX: OHorn-gly-ser-OHorn-ser-gly-OHasp. The nature of the acyl groups is presently under investigation. (2) 4-Hydroxy-2-nonenal (HNE), one the cytotoxic aldehydes produced by lipid-peroxidation, is noted for its ability to modify proteins at certain amino acid residues. Upon prolonged incubation of glucose-6- phosphate dehydrogenase (GLU-6-PDH) with HNE, the formation of cross- linked protein was observed. In order to understand the chemistry of the modification, the reaction between HNE and N-acetyllysine was studied. Three products, A, B and C, were detected in this reaction. On the basis of detailed mass spectral analyses of A and B after sodium borohydride reduction, B was assigned the structure of a Michael adduct derived from HNE and N-acetyllysine. A was shown to be the Schiff base formed from the Michael adduct with another molecule of N-acetyllysine. Thus the structure of A accounts for the cross-linking process. When GLU-6-PDH was modified by HNE followed by reduction with sodium borotritide, HPLC analysis of the acid hydrolysate showed two tritium peaks coeluting with A and B. Therefore, HNE reacted with lysine residues in the protein in the same manner as with N-acetyllysine. The structure of C is presently under investigation.